“Clearly location is not yet mainstream — it’s still a younger-demographic phenomenon — but if anyone can change it, Facebook will,” said Sam Altman, chief executive of Loopt. … For now, many people say sharing their physical location crosses a line, even if they freely share other information on the Web….

“I don’t like broadcasting where we are or when my husband’s gone, just for safety reasons,” said . And privacy concerns aside, she doesn’t see the need: “We go to playtime, the park and the grocery store. My life isn’t exciting enough to broadcast where I am and what I do.”

Location services are catching on more quickly with young people, who have grown up posting personal information online. “The magic age is people born after 1981,” said Mr. Altman of Loopt. “That’s the cut-off for us where we see a big change in privacy settings and user acceptance.”